For the clarity of the text, electro-reactions refer to different electro-catalytic phenomena including electro-coagulation, often referred to EC in the art.
Technologies based on electro-reactions are mostly used in the field of water and wastewater treatment. Contrary to other physico-chemical processes for coagulation for example, ERU avoids the use of chemical additives for coagulation which present a risk of water contamination by the other components of the additives (e.g. excess of sulfates and/or chlorides). Moreover, ERU eliminates the need for chemical products stocking, management and handling.
Basically an ERU comprises one or more electrolytic cells. A typical electrolytic cell is the arrangement of electrodes, one anode and one cathode. Electrical current (AC or DC) will cross the electrolyte (e.g. wastewater) from the anode to the cathode. In the case of multiple cells depending on the current alimentation of the cells, different configurations are possible. The electric power circulating between the two electrodes of the electrolytic cell submerged in the wastewater generates, depending on the electrodes material, corrosion of the anode, wastewater decomposition by electrolysis and other electro-catalytic phenomena such as oxidation. These phenomena will produce a multitude of reactive species into the polluted water. Those reactive species will give rise to a certain number of reactions such as coagulation, flocculation, oxidation, reduction, neutralization and others, depending on the water composition and the operational conditions. Thanks to those reactions generated in ERU, a large range of contaminants, from bacteria, viruses, phosphorous, fluoride, heavy metals, oils, grease, arsenic, etc. can be removed from wastewater. In fact, once the reaction phase is completed, the so formed byproducts (coagulated, flocculated, destabilized or neutralized substances) must undergo a separation process in order to complete the pollutant removal.
It is possible to control the reactions happening in the ERU by controlling some of the key parameters such as the electrodes material and the current density. Within an ERU, some pollutants are specifically removed by coagulation and flocculation (e.g. phosphorous) while some others are mostly removed by oxidation and other reactions (e.g. bacteria). While the production of species for coagulation need consumable anode material (e.g. iron or aluminum), species for oxidation are favored by non consumable anode material (e.g. boron doped diamond, etc.). Attempts have also been made to combine two different kinds of electrode material in the electrolytic cells in order to broaden the range of contaminants to be removed. For example, US 2008/0223731A1 describes and claims a combination of electrolytic cells with sacrificial anodes and electrolytic cells with non sacrificial anodes so as to form a more complete hybrid ERU. An example of controlling the ERU reactions by controlling the current density is given by researchers as Guohua Chen in “Separation and Purification Technologies”, vol. 38 (2004) pp. 11-41″. They have suggested a specific current density range of 2 to 2.5 mA·cm−2 as optimal for generation of species for coagulation and flocculation with consumable electrodes. Meanwhile, other studies such as V. Schmalz and collaborators in “Water Resources”, vol. 43 (2009), pp. 5260-5266, describe that in order to obtain good performance in generating species for oxidation, the current density must be set over 2.5 mA·cm−2.
The counter-effect of all ER process is the covering of the cathode electrode by an oxide layer of the corresponding anode metal and by ions, salts or biological foulants naturally present in the wastewater or a mixture thereof. In ERU the formation of an oxide layer blocking the electric current flow will be referred to as passivation phenomenon enhanced by other inorganic or organic foulants. The passivation phenomenon is directly related to the current density, the higher the current density, the higher the production of ions and oxide and consequently the higher the passivation phenomenon. The passivation phenomenon is limiting and explains why researchers as the ones cited above tend to give optimal values for current density in order to compensate between performance and passivation problems. Because of this passivation phenomenon, the electrodes need to be changed or cleaned periodically to function properly. This represents a challenge, particularly when ERU is chosen for a wastewater treatment facility where interventions are preferably limited. There is thus a need for a cost-effective ERU capable of continuous operation with few maintenance requirements.
Various automated cleaning systems have been proposed but most of them are inefficient and very expensive to run. For example, the Chinese patent CN 01108767.6 describes and claims a cleaning device comprising a wiper to remove deposits from the electrodes surface. This cleaning device is suitable for removal of organic foulants but fails to remove oxides layers which form on the surface of the electrodes. Another example is the Electrode Surface Activator (ESA) which is described and claimed in the American patent US 2008/0223731A1. This ESA makes use of a plurality of wipers to keep the electrodes from passivation. The ESA is considered as complicated and expensive. Previously known automated cleaning systems, including systems using polarity inversion or brushing/wiping functionalities, have been found insufficiently efficient or too expensive to provide a sustainable solution to fight the continuous formation of the oxide layer and other inorganic and organic foulants on electrodes. The most effective cleaning processes require a manual cleaning intervention with a shutdown of the ERU and are thus an expensive solution. Those cleaning devices and processes are also more appropriate for industrial or municipal wastewater treatment systems, which are often operated in batch and maintained on a daily basis by dedicated staff, and designed, with many treatment trains in parallel, allowing interruption of flow rate in each treatment train for maintenance purposes. On the contrary, smaller wastewater treatment systems are not operated by permanent staff and are often designed with one treatment train. These previous systems will be economically viable only if their maintenance requirements are kept at a minimum. The automation of the ERU is thus important for industrial and large municipal wastewater treatment systems, and even more important in the case of smaller wastewater treatment systems by reduction of maintenance requirements.
Thus, as can be appreciated, there is still a need for wastewater treatment improvement using self-cleaning device. This need is even more felt for non-industrial domestic wastewater treatment systems.